ANALYTICAL INVESTIGATIONS ON SUPER-SYNCHRONOUS OPERATION OF STATIC KRAMER DRIVE

Abstract

The Principle of slip power recovery has been employed over years in very large power induction motor drives to achieve variable speed operation at high efficiency. A static Kramer drive is essentially a constant torque type of slip power recovery drive in which the motor is of wound rotor type. Slip power, which is available at the rotor slip rings, is made to flow in a slip recovery loop consisting of a three phase bridge converter which is connected to slip rings on its ac side and another three phase bridge converter which is connected to supply mains on its ac side, the, two converters being connected to each other in cascade on their dc sides through suitable filtering inductor. Both converters are of line commutated type. If motor side converter is operated with zero firing delay, as a bridge diode recifier, and the motor side converter is operated with firing angle varied in a range 900 to 1800, the drive can be operated at varying speeds in sub-synchronous region. With the same arrangement of firing, it can also be operated as line excited generator at super-synchronous speeds. On the other hand, super-synchronous motoring and sub-synchronous generating modes are possible to be realized if the motor side converter is operated as a fully ON inverter (a = 1800), and the line side converter is operated as a controlled recifier in a firing angle range of 00 to 900. In present work, a general mathematical model of the static Kramer drive has been developed which is suitable for investigating steady-state and transient performance' of the drive in any of the four modes of operation mentioned above. The technique of d-q transformation of variables is employed with associated assumptions. Performance curves of the drive are obtained through digital simulation for all the four modes of operation to investigate the steady state behaviour. It is concluded that for large power applications static Kramer drive offers a robust arrangement which can be operated over a wide speed range in both motoring and regenerating mode. (